1. Field of the Invention
The present invention relates to a system which controls a pulse width of a pulse output from a pulse output circuit. More particularly, it relates to a system which can set a light-emitting pulse width of a laser diode to a required pulse width.
2. Description of the Prior Art
As various systems, in which digital signals are handled, have been widely used in recent years, an optical digital signal transmission system have been used for subscriber lines as well as primary lines. Accordingly it is necessary to realize a system for controlling a light-emitting pulse width in analogue, of a laser diode, especially, in a transmitter of a optical digital transmission system.
Increasingly, burst generated digital signals are directly processed due to a diversification of transmission systems. Therefore, it also becomes necessary to realize pulse widths control system which can control pulse width at a high speed, even when a mark to space ratio is widely varied.
Problems caused by pulse width variation generated in the beginning of a light-emitting pulse of a laser diode in a transmitter or by pulse width variation generated because of temperature and source-voltage variation will be now considered.
FIG. 22 shows characteristics of a known laser diode. A current I.sub.O indicates an output light to a driving current I.sub.B. Further, a current characteristic of the output current I.sub.O to the driving current I.sub.B that a light-emission threshold current Ith is varied according to an environmental temperature. It is apparent from FIG. 22 that the current characteristic is changed to a, b and c corresponding to the change of the environmental temperatures 0.degree. C., 25.degree. C. and 60.degree. C., respectively. Further, Y.sub.0, Y.sub.25, Y.sub.60 are inclination coefficients of the current characteristics a, b, and c, respectively. As the temperature becomes higher, the inclination becomes larger.
Judging from the current characteristic of the laser diode, even if the light-emission threshold current Ith is varied according to a change of temperature, the width of an output light-emitted pulse become narrower (see W.sub.0, W.sub.25 and W.sub.60 respectively corresponding to the temperatures 0.degree. C., 25.degree. C., and 60.degree. C.), even if a level of the light-emitted output respective to a data pulse DT is kept at constant value i.sub.0.
Thus, the light-emitting pulse width is varied according to the temperature. The variation of the light-emitting pulse width is caused by light-emission delay of a laser diode.
On the other hand, waveforms of an optical pulse for optical communications are prescribed in a standard of ITU-Recommendation. FIG. 23 illustrates a pattern mask of the eye diagram prescribed in the ITU-Recommendation G.662. Accordingly, it becomes necessary to control the variation of the light-emitting pulse width within a range of the eye pattern mask for optical communications.
As shown in FIG. 22, a bias current of the laser diode is controlled corresponding to the change of the temperature. That is, the bias current is changed to I.sub.B0, I.sub.B25 and I.sub.B60 as the temperature becomes 0.degree. C., 25.degree. C. and 60.degree. C. The bias current is set near about 80% of a threshold current.
In this way, the light-emitting pulse width becomes almost constant by controlling the bias current. However, the bias current is near the threshold current. Therefore, a dark light, called bias light-emission, is generated as noise.
Additionally, variable resistors are provided to a laser diode driving circuit in order to individually control for the beginning of the pulse variation due to the individual characteristic variation of the laser diode or the pulse variation due to source-voltage variation.
In recent years, a system for sending and receiving burst signals through an optical transmission path between a central office and a plurality of subscribers has been introduced. FIGS. 24A and 24B diagrams showing this system. Looking at FIG. 24A one station ST is connected to a star coupler COP through an optical transmission path, and further, the star coupler COP is connected to each of a plurality of subscribers 1 to n.
In this system, the station ST transmits burst signals downward and each of the subscribers transmits a response signal upward in return to the signal sent from the station ST. Distances from respective subscribers to the star coupler COP vary from each other.
Therefore, a signal level from a subscriber increase as a distance from the subscriber to the star coupler COP decreases. As described above, when the signal level increases, the bias light-emission becomes noise, and this noise is inputted to the star coupler COP.